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    In 1999, SGI added <span class="quote">“<span class="quote">concept checkers</span>”</span> to their
    implementation of the STL: code which checked the template
    parameters of instantiated pieces of the STL, in order to insure
    that the parameters being used met the requirements of the
    standard.  For example, the Standard requires that types passed as
    template parameters to <code class="classname">vector</code> be
    "Assignable" (which means what you think it means).  The
    checking was done during compilation, and none of the code was
    executed at runtime.
   </p><p>
     Unfortunately, the size of the compiler files grew significantly
     as a result.  The checking code itself was cumbersome.  And bugs
     were found in it on more than one occasion.
   </p><p>
     The primary author of the checking code, Jeremy Siek, had already
     started work on a replacement implementation.  The new code was
     formally reviewed and accepted into
   <a class="link" href="http://www.boost.org/libs/concept_check/concept_check.htm" target="_top">the
   Boost libraries</a>, and we are pleased to incorporate it into the
   GNU C++ library.
 </p><p>
   The new version imposes a much smaller space overhead on the generated
   object file.  The checks are also cleaner and easier to read and
   understand.
 </p><p>
   They are off by default for all versions of GCC.
   They can be enabled at configure time with
   <a class="link" href="configure.html" title="Configure"><code class="literal">--enable-concept-checks</code></a>.
   You can enable them on a per-translation-unit basis with
     <code class="literal">-D_GLIBCXX_CONCEPT_CHECKS</code>.
 </p><p>
   Please note that the checks are based on the requirements in the original
   C++ standard, many of which were relaxed in the C++11 standard and so valid
   C++11 code may be incorrectly rejected by the concept checks.  Additionally,
   some correct C++03 code might be rejected by the concept checks,
   for example template argument types may need to be complete when used in
   a template definition, rather than at the point of instantiation.
   There are no plans to address these shortcomings.
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